In many, if not most, glass-making operations it is highly desirable that molten glass issuing from the bowl of a forehearth be at a predetermined gob temperature for optimum molding or forming into desired articles such as bottles, jars and the like. As a direct result of nonuniformity in gob temperatures there can be (a) uneven distribution of the glass in the sidewalls of containers or bottles; (b) undesirably low wall strength and pressure resistance under testing; (c) poor surface finishes and nonuniform optical properties of the molded glass articles; (d) unduly slow operation of the molding and forming machines seriously affecting the economics of the operation; (e) practical problems involving lack of temperature uniformity between gobs in double and multiple gob operations; and (f) imprecise control of the weight of gobs in the continuing series of gobs being formed.
It is generally recognized that the genesis of the problems lies in the manner heat is removed from the molten stream in the forehearth cooling zone, most of which occur through heat absorption at the floor or bottom and side walls of the forehearth channel. Because these losses are not particularly controllable, the usual temperature homogeneity of the glass inflowing from the furnace is wierdly disrupted as it begins its flow through the forehearth. Nevertheless, it is necessary that the glass be cooled and conditioned to bring it down to the final gob temperature which is materially lower than the temperature prevailing in the furnace. The usual practice has been to allow the temperature to drop slowly from furnace to orifice in a gradually descending curve, the drop being more or less empirically controlled by above-surface and some sub-surface heating of the molten stream. As a result of sub-surface heating, minor Joule-effect currents are generated within the glass stream which tend to produce some slight agitation and, hence, a degree of mixing or blending of the marginal cooler and hotter central portions of the glass. It has been observed that while such manipulations of the glass stream tend to improve the operation to a slight degree, they fall short of completely eliminating the difficulties. One solution attempted has been the addition of a ring electrode in the vicinity of the bowl discharge orifice, however without full or satisfactory homogeneity in the discharged gobs, because often there are super-heated streaks in the gobs caused by the ring heaters themselves. It has therefore been the principal object of this invention to provide a method of operating a forehearth in such manner that the necessary temperature adjustment that must be accomplished in the forehearth channel between the furnace and the discharge orifice is fully controllable to produce gobs of improved uniformity around their surface, to increase forming speeds, to reduce the waste from defective articles due to poor surface and unequal wall thicknesses, and to improve the optical nature of articles formed.